4.1: Motion

The wings of this hummingbird are moving so fast that they’re just a blur of motion. You can probably think of many other examples of things in motion. If you can’t, just look around you. It’s likely that you’ll see something moving, and if nothing else, your eyes will be moving. So you know from experience what motion is. No doubt it seems like a fairly simple concept. However, when you read this article, you’ll find out that it’s not quite as simple as it seems.

Defining Motion

In science,
motion
is defined as a change in position. An object’s position is its location. Besides the wings of the hummingbird in opening image, you can see other examples of motion in the
Figure
below
. In each case, the position of something is changing.

Q
: In each picture in the
Figure
above
, what is moving and how is its position changing?

A
: The train and all its passengers are speeding straight down a track to the next station. The man and his bike are racing along a curving highway. The inchworm is slowly inching its way along a branch. The meteor is shooting through the atmosphere toward Earth, burning up as it goes.

Frame of Reference

There’s more to motion than objects simply changing position. You’ll see why when you consider the following example. Assume that the school bus pictured in the
Figure
below
passes by you as you stand on the sidewalk. It’s obvious to you that the bus is moving, but what about to the children inside the bus? The bus isn’t moving relative to them, and if they look at the other children sitting on the bus, they won’t appear to be moving either. If the ride is really smooth, the children may only be able to tell that the bus is moving by looking out the window and seeing you and the trees whizzing by.

This example shows that how we perceive motion depends on our frame of reference.
Frame of reference
refers to something that is not moving with respect to an observer that can be used to detect motion. For the children on the bus, if they use other children riding the bus as their frame of reference, they do not appear to be moving. But if they use objects outside the bus as their frame of reference, they can tell they are moving. The video at the URL below illustrates other examples of how frame of reference is related to motion.

Q
: What is your frame of reference if you are standing on the sidewalk and see the bus go by? How can you tell that the bus is moving?

A
: Your frame of reference might be the trees and other stationary objects across the street. As the bus goes by, it momentarily blocks your view of these objects, and this helps you detect the bus’ motion.

Summary

Motion is defined as a change of position.

How we perceive motion depends on our frame of reference. Frame of reference refers to something that is not moving with respect to an observer that can be used to detect motion.

Vocabulary

frame of reference
: Something that is not moving with respect to an observer that can be used to detect motion.

motion
: Change in position.

Practice

Do the frame of reference activity at the following URL. Watch the introduction and then do the nine trials. Repeat any trial you answer incorrectly until you get the correct answer.